In over the highway trucks and tractors it is now conventional to provide dual or tandem rear axles. In a majority of cases, at least in over the highway trucks and tractors used in North America, the rear axles are interconnected by an interaxle differential and both axles provide traction. The major advantage of driven tandem axles is that when ascending steep grades and when surface conditions are slippery the typical vehicle equipped with dual wheels has eight driven wheels to provide traction.
Other over the highway trucks with tandem rear axles have been equipped with one driven axle and a non-driven or tag axle. The advantage of the single driven axles versus dual driven axles is the elimination of the inter axle differential, a drop gear set and a rear axle ring and pinion. This results in increased fuel economy due to reduced friction and reduced weight.
Vehicles with adjustable loading on the driven axle, to enhance driven axle traction when desired, have been proposed. According to this proposal the driven axle is supported by leaf springs while the tag axle is supported by air springs. An air pressure control leveling valve is provided. The leveling valve is actuated by an operating rod which carries a roller that contacts the leaf spring. During normal operation this leveling valve functions to supply air under pressure at appropriate pressures to the tag axle air springs to maintain the vehicle at a desired level. When enhanced traction is desired, a manually operated change-over valve closes the leveling valve to air spring connection and vents the air spring to atmosphere to reduce the pressure in the air spring. A problem with this proposal is that the ability to level the vehicle to maintain a desired height is lost. Another problem is there is no provision to be certain that the driven axle is not overloaded. A further problem is the extent of tag axle air spring venting is controlled by a valve and as such the precision of air pressure control is a function of the condition of the control valve.